Unexpectedly, many chondrites, of every type, contain secondary minerals that formed as the primary mineral reacted with liquid water. The reactions took place soon after the chondrules formed and often during accretion of the incorporating bodies (Doyle et al. 2015), sometimes even before accretion. Water molecules suspended in space were wetting the grain surfaces and making them stickier, accelerating the process by which the grains coalesced. Carbonaceous chondrites tend to be particularly water-rich, possibly because they formed beyond the ‘snow line’ within which the infant Sun’s heat would have prevented water from condensing.
Aqueously altered minerals have also been detected in comets, the presence of liquid water disproving the notion that comets never got warm enough to melt their icy interiors (Berger et al. 2011). In the case of the asteroid Ryugu the hydration was dated to within the first 1.8 Ma of solar system history (McCain et al. 2023). Ceres, the only asteroid large enough to be rounded by its own gravity, is essentially a stationary comet, with half of its volume estimated to be ice. Occasionally cryovolcanoes erupt salty, ammonium-rich water from the interior.
Interplanetary space was wet. Evidence for this doesn’t just come from asteroids and comets. All the terrestrial planets show signs of having been drenched by water – even Mercury. One of the most astonishing findings of the Messenger mission was that under the beshadowed walls of Mercury’s high-latitude craters water abounds. Deposits 50 m thick are inferred to lie beneath the rims.
No less surprising is the evidence from Venus. It is so hostile to life that one might almost think it exists purposely to demonstrate that an Earth-like planet is not an inevitability. Although Venus today is dry, 465° C at the surface and shrouded under clouds of carbon dioxide and sulphuric acid, the high ratio of deuterium to hydrogen in its atmosphere implies that it once hosted a substantial ocean, subsequently evaporated or blasted away. Deuterium is an isotope of hydrogen, heavier than the ordinary form because the nucleus includes a neutron. It combines with oxygen to produce a heavy form of water. When ultraviolet radiation from the Sun split the evaporated water into hydrogen, deuterium and oxygen, the lightest gas, hydrogen, and most of the deuterium escaped into space. Some of the deuterium remained in the atmosphere, while the oxygen oxidised the crust. Dissociation from atmospheric HCO would be another mechanism of hydrogen loss.
Until quite recently, the Moon was believed to be devoid of water, but in October 2009 the LCROSS mission discovered significant quantities of water when it crashed part of the spacecraft into a crater close to the permanently shadowed south pole. Five months after that, it was announced that millions of tons of ice lay hidden in craters around the north pole. However, because of the difficulty of understanding where it might have come from, scientists continued to doubt that water was present. Conseequently it was still newsworthy when in 2018 a paper analysing data gathered by India’s Chandrayaan-1 probe gave the first definitive proof of surface ice. The presence of water is doubted no longer. Much of the water is likely to be a product of the solar wind (Xu et al. 2022); that at the poles is undoubtedly ancient.
Significant amounts have even been found in fragments of rock from beneath the surface, for example, in olivine crystals that grew while the containing volcanic melt was as yet unerupted (Hauri et al. 2011) and in apatite crystals that formed 4.4 Ga ago when the crust was pounded by asteroids (Tartèse et al. 2014). Analysis of the younger basalts recovered by China’s Chang’e-5 mission suggests that by 2.0 Ga the interior was essentially dry (Hu et al. 2021).
Water covers most of the Earth’s surface, to an average depth of almost 4 km. According to the nebula hypothesis, Earth should not have had oceans to start with, since interplanetary water at 1 AU would have been in vapour form and a putative global ‘magma ocean’ would have driven off whatever water might have been delivered by early-forming comets. Yet water has been abundant on or in the Earth from as far back as datable minerals can take us, in geological time as early as 4.4 Ga ago. Where this water came from continues to perplex. At the beginning of the Archaean 4.0 Ga ago, the entire planet is thought to have been under water.
Mars’s early history is also puzzling. Today its surface is cold, but like the other rocky planets it has an igneous crust, entailing that the surface once was molten. Evidence from a Martian meteorite suggests the crust formed surprisingly early: ‘no later than 4547 Ma’ (Bouvier et al. 2018) or 15 Ma after the formation of the planet itself, based on modelling. Not much more than 70 Ma after that, Mars was struck by asteroids. As the evidence comes from only one meteorite, we cannot categorically say that this was for the first time, but it is unlikely to have been an isolated occurrence. Mars’s surface is peppered with ancient craters.
Wherever one looks there is evidence of former water. The depression in its northern hemisphere once contained an ocean over 400 metres deep, covering a third of Mars’s surface. Formerly water-conducting deltas and valley networks fringe the basin. In other regions, splashes of sediment around impact craters suggest that the ground already had a mud-like consistency: the surface was wet when asteroids bombarded the planet. Thereafter, clouds continued their cycles of evaporation, re-precipitation and runoff as the water gradually seeped into the ground. As the water reacted with iron in the rock, the planet rusted.
Approximately 1 bar atmospheric pressure was needed to maintain liquid water on the surface, the same as on Earth (Palumbo et al. 2018). High levels of CO2 could have built up as a result of high rates of volcanic outgassing. Today the planet is colder, has only a 7 mbar atmosphere (nearly all CO2), and what remains of the water, liquid as well as frozen, is locked up beneath the surface and in large permanent ice caps around the poles.
Jupiter mostly consists of hydrogen and helium. The helium is best understood as a product of in-situ nuclear fusion, though the process is not now going on. At the equator, water makes up just 0.25% of the molecules in Jupiter’s atmosphere. The atmosphere also contains small amounts of oxygen, which combines with hydrogen to form H2O. At the equator, water makes up just 0.25% of the molecules in Jupiter’s atmosphere. As with the other giant planets, it also contains substantial amounts of ammonia (NH3) and methane.
Europa, one of the four moons that Galileo saw with his telescope, has a rocky interior with an icy shell; below the ice, an ocean of salty water is inferred. Charged particles streaming from Jupiter are continually splitting the frozen water into oxygen and hydrogen. Ganymede, the largest moon, is 50% water. Like Vesta, its rocky interior is differentiated, with a metallic core. Callisto’s low density suggests that it too is 50% water. Io, the innermost and densest of the four moons, is partly molten and has almost no water.
Saturn is best known for its rings, which mostly consist of ice particles. One of the rings is fed by vapour ejected from the moon Enceladus, which hides an ocean of water beneath its icy surface. Because the rings are undarkened by interplanetary dust, astronomers conclude that they must be relatively young. How they originated is not known. A collision between two icy moons is one possibility. Jupiter, Uranus and Neptune also have rings, but faint, unspectacular ones that evoke no wonder.
An ocean of water has also been discovered 20-30 km beneath the surface of Mimas, the smallest of Saturn’s regular moons. The surface is heavily cratered. Nonetheless, the ocean is inferred to be remarkably young, less than 15 million years (Lainey et al 2024). Titan is the second largest moon in the solar system after Ganymede and has a water-ice crust with lakes and rivers of liquid methane and ethane (C2H6). Liquid water lies beneath its surface, beneath that probably a mantle of frozen water and a rocky core.
The internal composition of these planets is unknown. Uranus could be a mixture of water and rock, with the water fraction anything from 33 ± 11% to 70 ± 17% (Morf et al. 2024). The innermost region could be 80% rock and the rest a mixture of hydrogen and helium, or there could be substantial amounts of carbon and nitrogen (Militzer 2024). Uranus’s moons are predominantly rocky. The four largest may have oceans beneath their icy crusts, with dissolved ammonia and salts acting as antifreeze.
Because of its pale greenish-blue colour (not ultramarine as in some images) Neptune was named after the Roman god of the Ocean, and its moons after minor water deities. Its atmosphere, primarily hydrogen and helium, makes up about 10% of its bulk. The colour comes from the absorption of red and infrared light by methane. Though similar in size, Neptune is 30% denser than Uranus, so probably contains more rock. Little is known about the moons’ composition, except for Triton.
the fragmentary nature of the TNOs – more than 100,000 objects over 50 km and trillions of objects 10–100 metres in size (Cooray 2006);- the low total mass of the Kuiper Belt objects (more easily determined than that of the Scattered Disc);
- the ‘surprisingly high level of dynamical excitation’ of the objects – that is, they have highly elliptical orbits at various angles to the ecliptic;
- the largest objects appear to be rocky.
The largest Kuiper Belt Objects (KBOs), Pluto, Haumea and Makemake, are around 70% rock, with a substantial ice component, and are classified as dwarf planets. Some of the smaller KBOs may be mainly ice. As with the asteroid belt, the vast number of such objects is thought to reflect collisions between larger bodies. Consequently the present state of the belt does not reflect its primeval state, and its more recent history may be one of disaggregation rather than aggregation.
The composition of KBOs large enough to be analysed is inferred from their reflected light. Interaction with polymer-producing cosmic rays and the Sun’s ultraviolet radiation has complicated their chemistry. In simple terms, the surfaces of the largest bodies are mainly water-ice, nitrogen and various carbon compounds. Smaller bodies are not cold or massive enough to have retained significant amounts of volatile ices. Pluto, about one third the volume of Earth’s moon, is one third water and two-thirds rock.
The ‘dynamical excitation’ of objects formerly in Pluto’s vicinity is evident from the thousands of impact craters dotting its surface. At least some of the smaller KBOs are fragments of larger ones and therefore younger than them. Some of the water ice is crystalline and must have formed in temperatures higher than those prevailing now. This may not have been long ago, since cosmic rays will reduce crystalline ice to an amorphous state within 0.1–1.0 million years.
What is true for water is also true for CO2: carbon dioxide ice is ubiquitous through the solar system, from the Kuiper Belt to Mercury’s beshadowed craters (De Prá et al. 2024). An explanation for this is not obvious.
Several pre-scientific peoples believed that a celestial ocean existed above the terrestrial one. The Egyptians visualised the Sun travelling through the heaven in a boat. Babylon’s creation myth, Enuma Elish, related that how Tiamat, a personification of the primordial waters, was split into an upper ocean and a lower ocean. The Hebrews, without personification, similarly maintained that the space containing the sun, moon and planets was the result of the Creator dividing waters initially created as one body.
Can ancient tradition and modern astronomical knowledge be reconciled? The Hebrew account suggests a protective, nebulous, circumambient shell not unlike the shape of the postulated Oort cloud. Because of the radiation from the Milky Way’s once luminous nucleus, interstellar space was warmer than today. The equivalent of a water layer 5 meters thick would have sufficed to absorb 90% of the radiation (Regener 1937).
Over time, the Sun’s gravity caused much of the vapour to diffuse inwards. By 4.57 Ga ago in geological time, interplanetary space may have hosted a substantial volume of water. Water requires surfaces termed ‘cloud condensation nuclei’ to make the transition from vapour to liquid, the most plentiful source of which would have been dust particles from the explosion of the largest rocky planets. Dust also would have come from the extant rocky planets when they were struck by the debris. Beyond Neptune, cooling and electrostatic sticking would have caused the droplets to consolidate into ice particles. The Kuiper Belt and Scattered Disc would be the mixed remains of water from the circumambient shell and rock from the outermost planets.
According to Genesis, the original Earth was destroyed in a terrible watery cataclysm. There were two agents of destruction. One was the springs of the great subterranean deep, which all at once exploded and flooded the planet. The other was the rain that for 40 days fell through ‘apertures of the heaven’. As a consequence of the rain, all life was blotted out.
At first sight, the rain is problematic. It is difficult to model an atmosphere that could have held that amount of water, and difficult to see how 40 days of rain, however torrential, could have drowned all the mountains and resulted in all life’s being obliterated. Genesis implies that up to that point rain was unknown: Earth’s surface was watered from below, not above. While some water will have evaporated from lakes and inland seas and from the ground itself, rain was not a major component of the hydrological cycle. On the other hand, some water must have reached the Earth from interplanetary space and lingered in the upper atmosphere. But in that case, what could have triggered its collapse?
Water might not have been all that rained on the planet. For example, in the story about Sodom and Gomorrah the text says that Yahweh rained sulphur and fire on the cities; the book of Joshua records him bringing down a hail of ‘large stones’, i.e. meteorites; Psalm 11 speaks of him raining down coals, i.e. volcanic bombs. The rain in Genesis might have been primarily a shower of asteroids. Vast amounts of dust kicked up by the impacts would have provided the nuclei for vapour in the upper atmosphere to precipitate as water. Water would also have been kicked up from the deep now flooding the land. As we shall discuss, the Earth is known to have been struck by asteroids in its early history. The only question is whether the impacts were spread out over millions of years – a bombardment in slow motion – or concentrated in one short episode. Heavy bombardment would have obliterated terrestrial life.
